1. Field of the Invention
The present invention generally relates to integrated circuit devices, and more particularly to an improved method for forming body contacts through the buried oxide region of a SOI semiconductor substrate.
2. Description of the Related Art
In buried layer devices known as silicon-on-insulator (SOI) devices, a buried insulation layer is formed beneath a thin surface silicon film. These devices have a number of potential advantages over conventional silicon devices (e.g., higher speed performance, higher temperature performance and increased radiation hardness).
SOI technology leads to an appreciable simplification of manufacturing processes, an increase in integration density, improved behavior under high voltages, and low sensitivity to radiation, since the volume of monocrystalline silicon is low. Inherent in SOI technology are the various techniques of ion implantation.
In one known technique of ion implantation, known by the acronym SIMOX, a very thin (0.1 micron-0.3 micron) layer of monocrystalline silicon is separated from the bulk of the silicon wafer by implanting a high dose of oxygen ions into the wafer to form a buried dielectric layer of silicon dioxide (having a typical thickness ranging from about 0.05 micron to 0.5 micron). This technique of xe2x80x9cseparation by implanted oxygenxe2x80x9d (SIMOX), provides a heterostructure in which a buried silicon dioxide layer serves as a highly effective insulator for surface layer electronic devices. Thus, this technology consists of implanting oxygen O sup.+ions or nitrogen N sup.+ions in heavy doses in solid monocrystalline silicon, so as to form, after high temperature annealing of the substrate, a buried insulating layer of silicon dioxide or silicon nitride.
As mentioned, one method for forming silicon-on-insulator (SOI) wafers is by xe2x80x9cseparation by implanted oxygenxe2x80x9d (SIMOX). Although the SIMOX method of implanting oxygen ions into silicon has been described in great detail, the electrical properties of the BOX (buried oxide) is generally such that it does not allow leakage of charge from the substrate to the SOI layer.
As SOI CMOS devices get smaller, the devices can suffer from a charge buildup in the body of the devices. This charge can cause a number of less than desirable effects, sometimes referred to as floating body effects. To ensure that specific devices do not suffer from these effects, a body contact is sometimes added as a method to drain off any charge in the body. The drawbacks of body contacts however, is increased size of the devices due to extra contacts. One alternative to front-side contacts is a bottom contact to the body. However, this requires making a contact through the BOX, directly under the body.
Current SOI substrates are used to form SOI devices exclusively, or the BOX depth is regionally altered so that both SOI and bulk devices can be formed on a substrate as taught in U.S. Pat. No. 5,548,149 which is incorporated herein by reference. Additionally, SOI current body contact methods require significant Si area and complex processing (e.g. photopatterning). This is due to the fact that an additional contact must be provided on the front surface of each body contacted device. Conventional processes provide using oxygen implants to form buried oxide layers. However, no process describes such a use in an SOI structure, or to do so in a simple process and for a smaller area. Moreover, the conventional processes are devoid of using such a process for electrical contacts through the BOX directly.
Thus, there is a need for an improved method of forming additional SO substrate contacts, which would reduce area and process complexity, and more specifically a SOI substrate wafer with an insulator having specific electrical properties for body contact applications, and for creating a leaky BOX in a controlled manner.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional SOI formulation and construction, the present invention has been devised, and it is an object of the present invention to provide a method for forming an improved silicon-on-insulator substrate wafer, and more specifically to create a silicon-on-insulator wafer with the insulator having specific electrical properties for body contact applications. It is another object of the present invention to use a leaky BOX in specific areas where the body contacts are desired. Still another object of the present invention is to induce a controlled amount of leakage to the BOX and also to produce this leakage in only specific areas of the substrate wafer. Yet another object of the present invention is to form a leaky BOX selectively on different parts of the substrate wafer.
In order to attain the objects suggested above, there is provided, according to one aspect of the invention a method of selectively modifying the buried oxide (BOX) region of a SOI substrate in order to form a leaky BOX, so as to form non-insulative areas of the BOX, thereby forming a resistive (yet conductive) substrate contact, and providing a method of forming the leaky BOX.
The present invention teaches a novel method for forming substrate contact regions on a SOI substrate without requiring additional space, and in order to provide lower diffusion capacitance. The method utilizes known semiconductor processing techniques. This method for selectively modifying the BOX region of a SOI substrate involves first providing a silicon substrate. Then, ion implanting the base O2 dose using SIMOX techniques (e.g. O2 implant) is accomplished. Next, SiO2 is deposited, followed by photopatterning to create a hard mask on the substrate to protect the modified BOX region. Then, a further ion implanting using a xe2x80x9ctouch-upxe2x80x9d O2 implant is accomplished, thereby resulting in a xe2x80x9cgoodxe2x80x9d quality BOX as typically practiced. The final step is to strip the hard mask followed by annealing the substrate. The area of the substrate, which had a hard mask present, would not receive the xe2x80x9ctouch-upxe2x80x9d O2 implant (second ion implant), which in turn would result in a xe2x80x9cleakyxe2x80x9d BOX.
In other words, the base dose oxygen implant itself creates a region of increased conductivity through the BOX structure. This region has an increased conductivity compared with regions of the substrate, which receive the base dose and xe2x80x9ctouch-upxe2x80x9d oxygen implant. The xe2x80x9ctouch-upxe2x80x9d implant, otherwise referred to as a room temperature implant, creates a non-conducting region in the BOX substrate structure, wherein this non-conducting region has a lower conductivity than the masked region. A mask is employed to block the second implant from specific areas causing a leaky BOX only in the areas defined by the mask.
Alternatively, the second O2 implant used in the BOX formation can be modified by implanting it at a different energy or dose level, such that the usual stoichiometric oxide cannot occur. Each of these techniques leads to a BOX layer that is deficient in the proper concentration of silicon and oxygen, thereby forming an electrically leaky BOX. This requires a slight modification to the process sequence previously described above, wherein the steps now comprise: First, providing a silicon substrate. Second, ion implanting the base O2 dose using SIMOX techniques (e.g. O2 implant) is accomplished. Third, further ion implanting using a xe2x80x9ctouch-upxe2x80x9d O2 implant is accomplished, thereby resulting in a good quality BOX as typically practiced. Fourth, a SiO2 layer is deposited followed by photopatterning to protect the good BOX region. Fifth, further ion implanting using a modified xe2x80x9ctouch-upxe2x80x9d O2 implant by using a different energy dosage or species to result in a xe2x80x9cleakyxe2x80x9d BOX in the open mask areas is accomplished. And finally, sixth, the hard mask is stripped off and the substrate is annealed.
Still, alternatively, the second O2 implant may be partially blocked by a hard mask such that the implant depth varies in order to make a xe2x80x9cgoodxe2x80x9d BOX in one region and a xe2x80x9cleakyxe2x80x9d BOX in another region. This sequence involves first providing a silicon substrate. The next step involves ion implanting the base O2 dose using SIMOX techniques (e.g. O2 implant). Then, an oxide or a nitride mask layer is deposited at a thickness suitable to provide a partial block of the O2 implant such that the peak concentration depth is changed. After which, the substrate is photopatterned. The next step is to etch the hardmask using a conventional reactive ion etching (RIE) process. Following this step is further ion implanting of the base using a xe2x80x9ctouch-upxe2x80x9d O2 implant, and finally the hard mask is stripped off and the substrate is annealed. The open mask regions can be either the xe2x80x9cgoodxe2x80x9d or xe2x80x9cleakyxe2x80x9d BOX region. Moreover, the xe2x80x9ctouch-upxe2x80x9d O2 implant energy is adjusted in order to achieve the desired result.
In another alternative method, the photopattern is the gate mask, wherein the processing would be as follows: As with the previous methods, a silicon substrate is provided. The next step is ion implantation of the base O2 dose using SIMOX techniques (e.g. O2 implant). Next, photopatterning of the substrate occurs in order to form the gate regions. Then, further ion implanting using a xe2x80x9ctouch-upxe2x80x9d O2 implant is accomplished, in order to result in a xe2x80x9cgoodxe2x80x9d BOX outside of the gate region, or in the source drain regions. In this step, the region under the gate photopattern would not receive the xe2x80x9ctouch-upxe2x80x9d O2 implant and thus would result in a xe2x80x9cleakyxe2x80x9d BOX region. The final step is annealing the substrate.
This alternative method allows for the modification of the BOX area in the source and drain regions to provide a full thickness BOX under the junctions in order to keep capacitance at a minimum. The second oxygen implant would not occur under the gate (channel region). Thus, this region would be deficient of oxygen, and therefore would remain electrically leaky. This leakage under the channel region would be resistive but would make an excellent body contact to the substrate. A single guard ring contact to the substrate through the BOX layer, which is commonly used on most SOI products, would serve to ground any charge built up during switching of this SOI device without any area penalty.